Laser induced breakdown spectroscopy (commonly known as “LIBS”) is a known process for detecting the chemical composition of a sample. LIBS involves focussing a laser on a target sample in order to generate sufficient heat to create ablation (i.e. material removal by vaporization of said material) of the target sample. As the sample material is initially ablated, plasma is created and emitted from the target sample at extremely high temperatures. The emitted plasma contains excited, disassociated atoms that emit light of a particular frequency when heated to a specific excitation temperature that can be detected and analyzed by a spectrometer in order to determine the chemical composition of the ablated sample.
More specifically, as the plasma cools, electrons from the various elements that comprise the sample fall from various excited states to lower energy states, emitting photons in the process. The frequency of these emitted photons is proportional to the energy of the photons and is, in turn, equal to the difference between the two energy states. The frequency (or its inverse, wavelength) and intensity of the photons can then be measured by a spectrometer-type detector to determine chemical composition of the sample spot where the plasma was created.
LIBS is typically minimally destructive, non-invasive and requires minimal sample preparation. It can be completed very rapidly and from a remote location, which makes it an excellent choice for applications where high volumes of analyses are required. LIBS can be used on any type of target sample and can detect any element on the periodic table.
However, known methods and apparatuses for conducting LIBS have a host of challenges and disadvantages. For example, many LIBS systems require relatively high powered lasers in order to effectively ablate the target sample. Further, as the target surface deteriorates the proper conditions for ablation may vary and many known methods are not well suited to adjusting the laser source in real time to maintain ideal ablation conditions. Finally, many known LIBS apparatuses are rather large and non-suited to applications requiring portable and compact instruments.
As will be readily understood by the skilled person, plasma creation is dependent mainly on the power density of the laser spot rather than the total overall power of the laser source. Therefore, a lower power laser must be focused to a smaller spot size to attain sufficient power density for plasma ignition. Therefore, in order to generate a sufficient energy density for plasma ignition in the sample region being analyzed, currently available low power lasers are typically focused down to a much smaller spot size than what would be required when using more powerful bench top lasers (i.e.: on the order of 5 μm to 100 μm).
It is therefore possible to use a much lower powered laser, however the main drawback of using lower power lasers is that the ablation area on the sample will be reduced accordingly as the laser post size is reduced, resulting in a more localized measurement, less plasma ablation and, as a result, less photons emitted from the sample. Therefore, detection is less reliable in these prior art arrangements.
Further, a small sample area (5 μm to 100 μm in diameter) creates additional problems that must be addressed when using a lower energy laser in practical applications. First, the laser must ideally be focused at the location where the analysis is required, which for most applications, is the surface of the sample.
However and as will be appreciated by the skilled person, minor deviation in the laser's focus results in inefficient and incomplete ablation at the sample surface, leading to incomplete plasma formation. As a result, the generated plasma is not representative of the actual sample being tested which can lead to erroneous analytical results. Also, it must be appreciated that in many real-world cases the samples being tested are not completely smooth or flat, exacerbating these complications.
A second issue that can present itself is sample cleanliness. LIBS is a very sensitive technique and the depth of the region being analyzed is typically just several microns deep across a sample area diameter of 5 μm to 100 μm. It can therefore be quite important that the surface being analyzed is truly representative of the sample and is therefore free of dirt, oils, oxidation and any other type of contamination. In prior art solutions, it is typical to fire a number of “cleaning shots” with the laser prior to analyzing the spectral data to determine composition. These cleaning shots can burn off superfluous material from the sample surface, permitting the analysis of the underlying clean material. For these cleaning tests to be effective, the laser must be properly focused on the contaminated sample surface.
A third issue that can present itself when using low powered lasers is that the sample can be inhomogeneous. Therefore, in some arrangements it can be required to fire the laser at several different locations on the surface of the sample and average the results.
One way that low power lasers can be used to create and maintain laser ablation is by varying the optimal distance of the focusing lens relative to the sample surface (i.e: the focal length of the focusing lens) as will be readily appreciated by the skilled person. These adjustments to the focal length are typically on the order of a few micrometers.
Available prior art solutions teach the concept of incrementally adjusting the focal length using, for example, a stepwise motor as taught in US Patent Publication Nos. 2014/0204375, 2014/0204377, 2014/0204376 and 2014/0204378. In these documents, the position of a focusing lens is varied in a stepwise, incremental manner based on output received from a spectrometer oriented to analyze the plasma emission. This spectrometer output is employed in what is effectively a feedback loop in order to adjust the position of the focusing lens relative to the sample in an incremental manner until the spectrometer output indicates a maximum or near maximum intensity of plasma formation. In short, the focal length (and therefore, efficiency of plasma formation) is adjusted based on the efficiency of the spectrometer operation.
This prior art approach is limited as adjusting the focusing lens through incremental movements and based on output from the spectrometer does not permit sufficient resolution and makes the process somewhat impractical as it can permit only rough evaluation rather than finely tuned measurement. Accordingly, a skilled person would appreciate that these prior art solutions are best suited to applications where the sample has relatively smooth surfaces (i.e.: flat polished samples) and is not ideally suited for use in connection with rough surfaces (mining samples, soil, oxide, metallic alloys etc.).
Accordingly, there is a need for an improved method and apparatus for performing laser induced breakdown atomic emission spectroscopy on a targeted sample in a compact format and using low power lasers.